1. Field of the Invention
The present invention relates to an apparatus for evaluating an image formation device, such as a laser printer and a copying machine. More particularly, the invention relates to a method of evaluating characteristics required of a light beam which is emitted from the write unit of an image formation device toward a latent image carrier, such as a photosensitive drum and a photosensitive belt. The invention also relates to a light beam characteristic evaluation apparatus that is employed for evaluating the characteristics, and further relates to an apparatus for adjusting a write unit by employing the evaluation method.
2. Description of the Related Art
Conventionally, an image forming device, such as a laser printer, a copying machine, and a facsimile device, performs writing on the surface of the photosensitive drum (latent image carrier) of an image forming unit by scanning the drum surface in both a horizontal scanning direction (i.e., main scanning direction) and a vertical scanning direction (i.e., sub-scanning direction) with a light beam emitted from a write unit, thereby forming an electrostatic latent image. In order to make the latent image visible and form a toner image, toner is caused to adhere to the surface of the photosensitive drum on which the latent image is formed. The toner image is transferred and fixed onto transfer paper. In this manner, an image is formed on the transfer paper.
The write unit is provided with an optical scanning system for scanning the surface of the photosensitive drum with a beam of light. The surface of the photosensitive drum is scanned in the horizontal scanning direction by the optical scanning system, while the surface is scanned in the vertical scanning direction by rotating the photosensitive drum.
In these image formation devices, incidentally, the characteristics required of the light beam of the write unit are evaluated in performing writing on the surface of the photosensitive drum which is a writing object.
For instance, in a copying machine, the image information on a manuscript is read in sequence and converted to a beam of light. In the case where the light beam writing position on the photosensitive drum surface deviates from a previously designed reference position, there arises a disadvantage that an image corresponding to the image information of a manuscript cannot be formed at the reference position. Particularly, in an image formation device, in which two laser light sources for emitting a beam of light are provided in the write unit and writing is performed on the photosensitive drum surface at two times the normal speed by scanning the photosensitive drum surface in the horizontal scanning direction concurrently with two light beams, if the writing position of one of the two light beams deviates from the writing position of the other light beam in the horizontal scanning direction, the image on a manuscript cannot be reproduced with high fidelity. Therefore, it is required to perform both the evaluation of the writing position of one light beam and the evaluation of the writing position of the other light beam.
In the case of a write unit which performs writing on a photosensitive drum by a single light beam, a writing position is computed for each surface of a polygon mirror constituting an optical scanning system. The object of evaluation in this case is both the position offset in the horizontal scanning direction (pitch fluctuation in the horizontal scanning direction) on each surface of the polygon mirror and the position offset in the vertical scanning direction (pitch fluctuation in the vertical scanning direction) on each surface of the polygon mirror.
In the case of a write unit which performs writing on a photosensitive drum by a plurality of multiplexed light beams, a pitch between light beams is also an object of evaluation in addition to the aforementioned evaluations.
Also, when two points on a manuscript in the horizontal scanning direction are extracted, two points on a copied image on transfer paper corresponding to the two points on the manuscript are extracted, and the distance between the two points on the manuscript is compared with the distance between the two points on the copied image, they must be equal to each other as long as copying is performed with equimagnification. If the distance between two points on a manuscript is not exactly equal to the distance between two points on a copied image, this will result in a magnification error. Since an image cannot be reproduced with high fidelity on transfer paper, performing the evaluation of a magnification error is required. In addition, in the case of enlargement and reduction, a ratio of a copied image formed on transfer paper to an image on a manuscript has to be equal to a desired magnification or demagnification ratio. If these ratio differ from each other, an image cannot be reproduced with high fidelity and therefore the evaluation of a magnification error will also be required.
Additionally, in the case where a left-side point and a right-side point on transfer paper are offset in the vertical scanning direction, it means that the scanning line has a tilt to the left or the right and therefore this scanning line tilt is also an object of evaluation.
Furthermore, assume that three points on a manuscript are extracted from left to right along the horizontal scanning direction and that the middle point is present at equal distances from the remaining two points. If the distance to the corresponding left-side point and the distance to the corresponding right-side point on the transfer paper are not equal with the corresponding middle point on the transfer paper as reference, a copied image will lack the balance between the right side and the left side. Therefore, it is also required to evaluate whether or not a distance from a middle point to a left-side point and a distance from a middle point to a right-side point are equal to each other.
In this case, if the difference between the writing position of the left-side point and the writing position of the middle point is not equal in the vertical scanning direction to the difference between the writing position of the right-side point and the writing position of the middle point, the scanning line will have a curve. Similarly, an image is not reproduced with high fidelity, so that it is also required to evaluate whether or not a scanning line has a curve.
Incidentally, a conventional apparatus for evaluating the characteristics of a light beam in the horizontal scanning direction is shown, for example, in FIG. 1 (see Japanese Laid-Open Patent Publication No. HEI 5-284293).
In the figure, reference numeral 1 denotes a write unit (optical unit). The write unit 1 is provided with a beam source (laser light source) 2, a rotatable polygon mirror 3, and an f .theta. lens 4. The beam source (laser light source) consists of a semiconductor laser 2. The semiconductor laser 2 is modulated and driven by an optical analog modulator 5. The optical analog modulator 5 modulates the strength of laser light emitted from the semiconductor laser 2 in correspondence to a manuscript image. The laser light emitted from the semiconductor laser 2 is deflected by rotation of the polygon mirror 3.
A pair of spaced photoelectric conversion elements 7a and 7b are provided in the horizontal scanning direction on a photosensitive surface 6 equivalent to the surface of a photosensitive drum provided in an image forming unit In order to enhance received-light position accuracy (writing position accuracy), light intercepting plates 8a and 8b with a pinhole (circular small hole) are provided directly before the photoelectric conversion elements 7a and 7b. Let the distance between this pair of pinholes be L.
If the polygon mirror 3 is rotated with the semiconductor laser 2 lit at all times during scanning and if the photosensitive surface 6 is scanned in the horizontal direction Q1 with the light beam Pi, the first photoelectric conversion element 7a will first receive the light beam P1 and then the second photoelectric conversion element 7b will receive the light beam P1. From the difference between the light receiving times and the distance L, an actual scanning speed of the light beam P1 of this write unit 1 can be computed. When this actually measured scanning speed of the light beam P1 is faster or slower than a previously designed scanning speed, the writing position of the light beam P1 is offset from the writing reference position.
Hence, whether this actually measured scanning speed of the light beam is within the allowable error of the designed scanning speed is evaluated. In the case where the measured scanning speed has exceeded this allowable error, the revolution speed of the polygon mirror 3 is adjusted so that the scanning speed of the write unit is within the allowable error.
This conventional light beam characteristic evaluation apparatus cannot compute the writing position itself directly. If it is to be computed, time needs to be computed until an output signal is output from the second photoelectric conversion element 7b since an output signal was output from the first photoelectric conversion element 7a. In addition, an actual scanning speed needs to be computed by dividing the distance L with the computed time, and a computation for converting this scanning speed to a writing position is needed. Therefore, the procedure for computing the writing position becomes complicated. Also, the characteristics of the light beam to be evaluated are limited.
Next, in the case where the beam diameter of the light beam P1 on the surface of the photosensitive drum is offset from a previously designed value, the edge of an image formed on transfer paper will become dim, or cracks will occur in the scanning line, so that there is a disadvantage that picture quality will be degraded. Therefore, it is also required to evaluate the diameter or shape of the light beam on the scanned surface.
In a conventional method of evaluating the beam diameter of a light beam, a pinhole or a slit is provided at a position corresponding to the surface of a photosensitive drum, and a light receiving device is provided directly after the pinhole or the slit. With this arrangement, the beam diameter is measured in a stationary state. This conventional method, however, cannot measure the beam diameter in a scanning state.
Hence, in order to measure the beam diameter in a scanning state, a method of and an apparatus for evaluating the beam diameter of a light beam have been proposed (see Japanese Laid-Open Patent Publication No. HEI 4-351928). As shown in FIG. 2, a one-dimensional (1-D) charge coupled device (CCD) 9 is provided on the photosensitive surface 6 equivalent to the surface of the photosensitive drum. In the optical path of the light beam P1 traveling toward the 1-D CCD 9, an objective lens is provided for directing the light beam P1 onto the photosensitive surface 6. While the beam spot S of the light beam P1 is being moved in a direction of arrow Q1 along the horizontal scanning direction, the 1-D CCD 9 is scanned n times in a direction of arrow Q2. The light quantity signals of pixels C1 to Cn during a signal scan are integrated and stored in a storage circuit. By computing a signal from this storage circuit, the light beam diameter is computed.
Incidentally, in this conventional evaluation method, when the 1-D CCD 9 is moved once in the direction of arrow Q2 and then is moved again in the direction of arrow Q2, the 1-scan period t1 of the 1-D CCD 9 has elapsed. For this reason, the light beam P1 has moved in the horizontal scanning direction (direction of arrow Q1) for this 1-scan period t1. Therefore, this evaluation method is equivalent to the constitution in which n 1-D CCDs 9 are arranged at regular intervals with the beam spot S in a stationary state, as schematically shown in FIG. 3.
In this evaluation method, as evident in FIG. 3, the light beam P1 has moved in the horizontal scanning direction for the 1-scan period t1 of the 1-D CCD 9, so that the beam spot S is fetched in a thinned-out state in the 1-D CCD 9. Furthermore, during the scanning period .DELTA.t between the time that after a certain pixel Ci of the 1-D CCD 9 is scanned, the image information is read and the time that after the adjacent pixel Ci+1 is scanned, the image information is read, the light beam P1 also moves in the horizontal scanning direction (direction of arrow Q1). Therefore, this is equivalent to fetching an image of the beam spot S by obliquely scanning the 1-D CCD 9, so that an error easily comes to occur when the beam diameter is quantized. The evaluation error in this quantization of the beam diameter is increased as the scanning speed of the light beam P1 is increased.
The aforementioned conventional light beam characteristic evaluation method (beam diameter evaluation method), therefore, has the disadvantage that it is difficult to enhance the evaluation accuracy of the beam diameter.
As previously described, the characteristics required of the light beam are a writing position characteristic to a photosensitive drum surface, pitch fluctuation in a horizontal scanning direction, pitch fluctuation in a vertical scanning direction, a beam-to-beam pitch, a magnification error, right-left balance (magnification error deviation), a scanning line curvature, a light beam diameter, a beam shape, and so on. In prior art, since the beam characteristics are evaluated by exclusive evaluation apparatuses, the characteristic evaluation of the light beam becomes complicated and is not a synthetic evaluation under the same condition, so that there is a fear that reliability of evaluation will be slightly reduced.
Furthermore, for a method of evaluating a beam spot diameter or a beam spot shape, there is a demand for an even greater enhancement in the evaluation accuracy of the beam spot diameter or beam spot shape in a scanning state.
In addition, positioning of a reference position is required in order to perform these evaluations.
For instance, Japanese Laid-Open Patent Publication No. HEI 8-86616 discloses a three-dimensional (3-D) image measurement apparatus equipped with a computer. This measurement apparatus is provided with a laser head for emitting a cross-shaped slit light to an object of measurement having a 3-D shape. The laser head is provided on a laser head bed so that it is rotatable on the intersection of the crossed slit and movable in a right-and-left direction and an up-and-down direction. This measurement apparatus is further provided with a CCD camera for photographing the measurement object, an image processing section for processing an image signal photographed by the CCD camera, and a laser head operation control section. In this 3-D image measurement apparatus, the lens center of the CCD camera and the center portion of the point end of the laser head are located on the X axis of a 3-D absolute coordinate system, and the photographing surface of the CCD camera is arranged in parallel to the X-Y plane.
This image measurement apparatus merely performs positioning of the area-type CCD element of the CCD camera by adjusting the area-type CCD element at a specific position in correspondence to the photographing position of the area-type CCD element and does not specify a reference pixel as a reference position for measurement. For this reason, there is a problem in that the offset between the positions of the reference pixel and laser light cannot be accurately grasped.